Process and apparatus for continuous fluid coating of a traveling web



March 24, 1970 MAMORU ISHIWATA ET AL 3,502,494

PROCESS AND AYPARATUS FOR CONTINUOUS FLUID COATING OF A TRAVELING WEBFiled Nov. 4, 1966 3 Sheets-Sheet 1 FIGI mvmons MAMORU ISHIWATA YOSUKEUCHIDA March 24, 1970 MAMORU ISHIWATA ETAL 3,502,494

PROCESS AND APPARATUS FOR CONTINUOUS FLUID COATING OF A TRAVELING WEBFiled Nov. '4 1966 3 Sheets- Sheec 2 4 9 i vT INVENTOR$ 4 Y MAMORUISHIWATA YOSUKE UCHIDA BY I AM wm March 24, 1970 MAMORU ISHIWATA ETAL3,502,494

- PROCESS AND APPARATUS FOR CONTINUOUS FLUID COATING OF A TRAVELING WEBFiled Nov. 4, 1966 3 Sheets-Sheet 3 P -P2 =00! Kg/cm t Flee (cc/ 200--0R Pl 'P2=0 100-- P -P2= -o.o|5 Kq/cm 0 I I I INVENTORS MAMORU ISHIWATAYOSUKE UCHIDA v d s United States Patent ()fice 3,502,494 Patented Mar.24, 1970 3,502,494 PROCESS AND APPARATUS FOR CONTINUOUS FLUID COATING OFA TRAVELING WEB 1 Mamoru Ishiwata and Yosuke Uchida, Ashigara-Kamigun,Kanagawa, Japan, assignors to Fuji Shashin Film Kabushiki Kaisha,Kanagawa, Japan Filed Nov. 4, 1966, Ser. No. 592,096 Claims priority,application Japan, Nov. 4, 1965, 4 67,667 Int. 'Cl. B05c 3/12 U.S. Cl.117-34 7 Claims ABSTRACT OF THE DISCLOSURE Process for coating atraveling web utilizing a doctor blade wherein improved results areobtained by reducing the pressure on the fluid coating composition in afluid coating composition holding chamber, and simultaneously reducingthe pressure on the backside and transverse sides of the composition asit is drawn from the doctor edge. A thinner coating is withdrawn thanotherwise obtainable.

Apparatus for forming the above process comprising means to reduce thepressure on the fluid coating composition, a vacuum chamber surroundingthe back and transverse sides of the coating composition is alsodisclosed. The means of reducing the pressure on the fluid coatingcomposition comprises means for evacuating the space above thecomposition in the chamber containing the composition.

The present invention relates to a process for coating continuously afluid coating composition or fiuid coating compositions in a layer orlayers on the surface of a continuously traveling web and moreparticularly to an improved so-called doctor coating process or knifecoating process by retaining a fluid coating composition in the chamberdirectly behind the doctor edge of a coating means placed with a slightinterval from the surface of a continuously traveling web to be coatedand withdrawing said coating composition therefrom by the traveling webin an amount defined by the clearance between the doctor edge and thesurface of the web.

In doctor coating or knife coating, the thickness of the coated layerwithdrawn by a traveling web may be generally represented by thefollowing equation referring to the relations shown in FIG. 6 of theaccompanying drawing:

wn-P2100 1 9 60S tc/2[1+ m t=thickness of the coated layer (cm),c=-clearance between doctor edge and web surface (cm.), l=length ofdoctor edge in the traveling direction of web (cm.), =viscosity of fluidcoating composition (gr./crn. sec.), =density of coating composition(gr./cm. p =presSure of coating composition at the entrance side ofdoctor edge (Gr/sq. cm.), p =pressure of coating composition at the exitside of doctor edge (Gr./ sq. cm.), Li -velocity of support or web(cm./sec.), 0=degree of angle between the upwardly directing verticalline and the traveling direction of web, g=gravitational acceleration(=981) (cm./sec. and g =gravitational conversion factor (:981) (gr. cm./

Gr./sec.

In an ordinary doctor coating process, Equation A becomes wherein (I1P2) clpg cos 020 and hence That is, the thickness of coating cannot bereduced to less than /2 of the clearance between the doctor edge and thesupport. Accordingly, in order to carry out thin layer coating by doctorcoating, the clearance between the doctor edge and the support must benarrowed, which is accompanied by the following three difliculties.

One of the difliculties is that the clearance between the doctor edgeand the support is locally narrowed when foreign solid matters depositedon or embedded in the back surface of the support or the surface of aroll pass through said clearance, which results in the formation ofcomets in the coated layer. The second difliculty is that accompaniedwith narrowing the clearance, the effect of errors in the accuracy ofthe doctor edge and the roll and in the thickness of the support areincreased, which makes the thickness of the coated layer uneven. Thethird difficulty is that in the case where the joint portion of supportspasses through the clearance betwen the doctor edge and the support, thecoating means or roll must be moved so that the clearance is enlarged.

The inventors have found an improved coating process and apparatus notaccompanied by the above-mentioned difficulties, which will be explainedreferring to the accompanying drawings, in which FIG. 1 is a sidecross-sectional view showing an embodiment of the present invention,

FIG. 2 is a side cross-sectional view showing another embodiment of thisinvention,

FIG. 3 is a side cross-sectional view showing still another embodimentof the present invention in which the principle of the invention isapplied to obtain multiple layers,

FIG. 4 is a side cross-sectional view showing another embodiment ofmultiple layer coating according to the present invention,

FIG. 5 is an enlarged sectional view showing a portion of a doctor edge,and

FIG. 6 is a graph showing the influence of the clearance between adoctor edge and the surface of a web on the amount of a fluid coatingcomposition coated on the web surface.

Referring now to FIG. 1, there is shown a coating apparatus for carryingout one embodiment of the present invention. A web 1 to be coated iscontinuously moved and backed up by a roll 2. A coating means 3 having adoctor edge 4 and a chamber 5 for containing a coating composition, saidchamber 5 being formed behind said doctor edge 4, is placed at a slightinterval from the surface of the traveling web. The coating compositionin the chamber is maintained at a definite level by means of a liquidlevel controlling means 6 and the pressure in the space 8 above thefluid composition in the chamber is reduced by means of a blower 7.Further, if necessary, a chamber 10, the pressure in which is suitablyreduced by means of a blower 9, is provided to the back side and thetransverse sides of the coating means to prevent the entrance of theambient atmosphere into the chamber 5. A fluid coating composition, suchas a photographic emulsion, is supplied into the chamber 5 by means of apump 11 at a rate controlled by means of the aforesaid level controllingmeans 6, and is withdrawn from the chamber by the traveling web toprovide the coated layer thereof on the surface of said web.

FIG. 2 shows another embodiment of the present invention, in which acoating means having a doctor edge 4 and a chamber 5 for a fluid coatingcomposition and further equipped with, if necessary, a vacuum chamber 10as in FIG. 1 is connected through a conduit 13 to a subsidiary chamberor tank 12. The tank 12 is equipped with a level controlling means 6 andhas a space 8, the pressure of which is maintained at a proper reducedpressure by means of a blower 7. The fluid coating composition at theentrance of the doctor edge may be maintained at the value satisfyingthe following relation by leaving the space 8 and reducing the pressurein said space;

(P1P2) cpg cos 0 Accordingly, it follows;

Thus, the thickness of the coated layer can be reduced to less than A2of the clearance between the doctor edge and the web surface. If thecoating procedure is conducted by the following conditions:

u=20 m./min.

p =1.013 kg./cm. (atmospheric pressure) 1.1.:50 cp.

The relation of the thickness of the coated layer and the clearancebetween the doctor edge and the web surface is illustrated in FIG. 6,which shows that the clearance for obtaining the thickness of the coatedlayer of, for example, 80 microns by an ordinary doctor coating method(p p ==0.01 kg./cm. is 0.15 mm., whereas that by the present invention(p -p =-0.0l5 kg./cm. is 0.47 mm.

Further, in the case of using the coating means of which the length ofthe doctor edge in the traveling direction of the web is 5 mm. and theclearance between the doctor edge and the web surface is 0.4-0.5 mm.,the pressure diiference between the entrance and the exit of the doctoredge, that is, 11 -11 in FIG. 5 may be 80 to 250 mm. H O gauge forcoating at a coating rate of 20-- 80 m./min. a coating compositionhaving 30-100 cp. in viscosity and 1.0-1.1 gr./cc. in density in athickness of 20-80 microns. Consequently, if the liquid level ismaintained at 20-50 mm. H O, the pressure in the space 8 is usually fromabout l00 to about 300 mm. H O gauge.

As is clear from the above-mentioned example, according to the presentinvention, thin-layer coating can be conducted with a comparativelylarge clearance between the doctor edge and the web surface and notaccompanied by such troubles as the formation of comets, unevenness ofcoating thickness, and the difiiculty encountered in passing the jointedportion of webs or supports through the clearance.

It will be understood from the above descriptions that an object of thepresent invention is to provide an improved coating process andapparatus by which thin coating of a good quality is obtained withoutnarrowing the clearance between the doctor edge and the support.Further, the feature of the present invention is in the point that byproviding a space having a reduced pressure above a fluid coatingcomposition in a chamber of a coating means or in a subsidiary chamberconnected to the coating means, a pressure gradient is applied to afluid coating composition positioned between the doctor edge and atraveling support such that the pressure is gradually decreased in thedirection opposite to the traveling direction of said support. Anotherfeature of the present invention besides the above system, is providinga chamber having a reduced pressure at the back side and the transversesides of said coating means, whereby the aforesaid pressure gradient canbe increased remarkably.

Moreover, in the case of applying the present invention to thesimultaneous coating of multiple layers, as illustrated in FIG. 3 andFIG. 4, a fluid coating composition or fluid coating compositions forforming a second or more layers on the coated layer on a support may becontinuously supplied to the fore-wall of the doctor edge which isinclined toward the doctor edge and flow down in a layer or layers alongthe surface thereof. Also, an extrusion type hopper or extrusion typehoppers for forming a second layer or more thereon may be provided tothe fore-wall of the doctor edge and said coating compositions may beapplied to the coated surface on the support without leaving any airspace between the layers.

The invention will be further explained by the following examples,although the invention shall not be limited to them.

EXAMPLE 1 By employing the coating apparatus illustrated in FIG. 1, agelatino silver halide photographic emulsion for positive photographiccinefilm was applied to the surface of an undercoated cellulosetriacetate film in accordance with the present invention. The operatingconditions were as follows:

Coating composition: Viscosity 30 cp., density 1.05 g./cc.

Clearance between the doctor edge and the support: 0.5

Velocity of traveling support: 40 m./ min.

Liquid level of the coating composition in chamber 5:

Pressure in reduced-pressure chamber 8: 175 mm. H O

gauge Pressure in reduced-pressure chamber 10:

mm. H O gauge.

The thickness of the thus coated layer was about /5 of one obtained by aconventional doctor coating. After coating, the coated layer wasimmediately set and then dried, and the resulting film was subjected toquality tests, which showed that the layer had a uniform thickness andno comets were formed therein.

EXAMPLE 2 Three layers of gelatino silver halide photographic emulsionsfor positive color cinefilm were applied on an undercoated cellulosetriacetate film using the coated apparatus illustrated in FIG. 4 inaccordance with the present invention. The operating conditions were asfollows:

First coated layerBlue-sensitive emulsion layer: Viscosity 50 cp.,density 1.05 g./cc.,

Second coated layerRed-sensitive emulsion layer: Viscosity 50 cp.,density 1.03 g./cc.,

Third coated layerGreen-sensitive layer: Viscosity 50 c.p. density 1.04g./cc.,

Clearance between doctor edge and support: 0.4 mm.,

Flow rate of the second coating composition: 1.2 liter/ min./m.-width,

Flow rate of the third coating composition: 1.0 liter/ min./m.-width,

Velocity of traveling support: 20 m./min.,

Level of the first coating composition in subsidiary tank Pressure inreduced-pressure chamber 8: -225 mm.

H O gauge,

Pressure in reduced-pressure chamber 10: -l to 230 mm. H O gauge.

The thickness of the thus coated first layer was about one-fourth thatof a layer obtained by a conventional doctor coating method. Aftercoating, the coated layers were immediately set and dried, and theresulting film was subjected to quality tests, which showed that thelayers have a uniform thickness, that no comets \were observed in thelayers, and that no intermixing of coating compositions was observedbetween layers.

Although the invention was explained with reference to specificexamples, it will be understood that it is not limited to the conditionsfor operating the coating procedure as well as the kind of coatingcompositions and the configuration of the coating means. Similarly,according to the present invention, the traveling direction of a supportat the doctor edge of a forming means and a system for conductingmultiple layer coating may be selected as desired.

What is claimed is:

1. In a process for coating 21 fluid coating composition on acontinuously traveling web by placing a coating means, having a doctoredge and a chamber containing a fluid coating composition directlybehind said doctor edge, at a slight interval from the surface of saidweb, retaining the fluid coating composition in the chamber, andwithdrawing said coating composition from said chamber by the travelingweb surface, the improvement which comprises reducing the pressure onthe fluid coating composition in said chamber and simultaneouslyreducing the pressure on the back side and the transverse sides of saidcomposition as it is drawn from said doctor edge by said web so that athinner coating is withdrawn than \would otherwise be withdrawn ontosaid web at the same interval between the doctor edge and web.

2. The method of claim 1 wherein the pressure on the fluid coatingcomposition in said chamber is reduced by reducing the pressure in aspace above the composition in said chamber.

3. The method of claim 1 wherein the pressure on the fluid coatingcomposition is reduced by reducing the pressure in a second chamberconnected to the chamber containing said composition so that thepressure on said composition is reduced.

4. The method of claim 1 further comprising applying at least oneadditional layer of fluid coating composition to the surface of thefirst coated layer at the exit side of the doctor edge withoutpermitting an a r space at the interface between the first coated layerand said additional layer.

5. The process of claim 1 wherein said fluid coating composition is acomposition for the production of photographic, light-sensitiveelements.

6. In an apparatus for applying a fluid coating composition to a webcomprising a doctor edge and a chamber for supplying said composition tosaid doctor edge and means for advancing a web which is to be coated ata small interval from said doctor edge, so that said composition iswithdrawn from said doctor edge at a layer on the surface of said web,the improvement which comprises:

means to reduce the pressure on said fluid coating composition beinglwithdrawn from said doctor edge, means comprising means for evacuatinga space above said composition in the chamber containing saidcomposition, and vacuum means surrounding the back and transverse sidesof said coating composition so that a thinner coating is withdrawn thanwould otherwise be withdrawn onto said web at the same interval betweenthe doctor edge and said web.

7,. The apparatus of claim 6 wherein said means to reduce the pressureon said fluid coating composition comprises a second chamber soconnected to the. chamber containing said composition that a reductionin pressure in said second chamber results in a reduction in thepressure on said composition and means to reduce the pressure in saidsecond chamber.

References Cited UNITED STATES PATENTS 3,206,323 9/1965 Miller et al.117-34 3,348,964 10/1967 Good 118-50 X ALFRED L. LEAVITT, PrimaryExaminer T. E. BOKAN, Assistant Examiner US. Cl. X.R.

